The Gregorian Calendar is a pure Solar Calendar, based only on the the dual movement of the Sun and the Earth. A solar year has 365 days or 366 for a bissextile year, ignoring the month (the Moon). While a Solilunar calendar, integrating the triple motion of Sun-Earth-Moon, just as the Chinese Calendar, takes the motion of the Moon in consideration, describes precisely the Month of about 29 days, a whole revolution around the Earth.
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